1. Field of the Invention
This invention relates to a reading head for the magnetic scanning of two rows of juxtaposed parallel Wiegand wires or other elongate bistable magnetic elements, which reading head comprises an E-shaped soft magnetic core having three parallel legs, which have free ends disposed in the reading surface of the reading head and are connected at their opposite ends by a backbone spaced behind the reading surface of the reading head, and an electrically conductive detector winding surrounding the intermediate one of said legs, said reading head also comprising two short permanent magnets, which are spaced behind the reading surface and have relatively large pole faces and have the same flux density at said pole faces, one of said permanent magnets being spaced above the uppermost leg of the E-shaped core, and the other of said permanent magnets being spaced below the lower leg of the E-shaped core, said permanent magnets having a direction of magnetization which is approximately parallel to the reading surface of the reading head, said permanent magnets being so arranged that the magnetic field established between them has substantially antiparallel directions of flux which are approximately parallel to said backbone on opposite sides of said core.
2. Description of the Prior Art
Such reading head has been described in prior Laid-open German Application No. 32 23 924 and serves to read information which has been encoded in a pattern of Wiegand wires or similar bistable magnetic elements.
Wiegand wires are ferromagnetic wires which have a homogeneous composition and consist, e.g., of an iron-nickel alloy comprising preferably 48% iron and 52% nickel, or of an iron-cobalt alloy, or of a cobalt-nickle alloy, or of a cobalt-iron-vanadium alloy comprising preferably 52% cobalt, 38% iron and 10% vanadium, and which have been subjected to a special mechanical and thermal treatment resulting in the formation of said wires with a soft magnetic core and a hard magnetic shell, which has a higher coercive force than the core. Typical Wiegand wires have a length of 10 to 50 mm, preferably of 20 to 30 mm. When a Wiegand wire which has been magnetized to saturation in a magnetic field having a magnetic field strength of at least 80 A/cm and preferably in excess of 100 A/cm so that the soft magnetic core and the hard magnetic shell are magnetized in the same direction and said Wiegand wire is introduced into an external magnetic field which has the same direction as the axis of the wire and said direction is opposite to the direction of flux in the Wiegand wire, in the direction of flux in the soft core of the Wiegand wire will be reversed when the magnetic field strength of said external magnetic field exceeds a value of about 16 A/cm.
That reverasl can also be called resetting. When the magnetic field is reversed once more beyond a critical field strengh of the external magnetic field (that critical field strength is called the triggering field strength), the direction of the flux in the core will be reversed again so that the core and the shell will again have the same directions of flux. That reversal of the direction of flux takes place very quickly and is accomplished by a correspondingly large change of the magnetic flux per unit of time (Wiegand effect). That change of the magnetic flux can induce a short and very strong voltage pulse (Wiegand pulse) of up to about 12 volts in an induction winding, which is called detector winding and the number of turns and the load resistance of which will determine the induced voltage.
A pulse will also be generated in the detector winding when the core is reset but that pulse will have a much lower amplitude than the pulse generated during the reversal form the antiparallel to the parallel direction of flux in the wire and will have the opposite sign. If the Wiegand wire lies in a magnetic field which is reversed from time to time and which is so strong that it can reverse the magnetization first of the core and subsequently also of the shell and effect satuartion in each case, the reversal of the direction of flux in the soft magnetic core will result in the generation of Wiegand pulses of positive and negative polarities in alternation. This is described as a symmetrical exciation of the Wiegand wire. For this purpose field strengths from about -(80 to 120 A/cm) to +(80 to 120 A/cm) are required. The reversal of the direction of flux in the shell takes place also suddenly and results in a pulse in the detector winding but that pulse is much smaller than the pulse induced by the reversal of the direction of flux in the core.
If the external magnetic field can reverse only the direction of flux in the soft core but cannot reverse the direction of flux in the hard shell, the strong Wiegand pulses which are generated will have the same polarity. This result is described as a asymmetric excitation of the Wiegand wire. For this purpose a field strength of at least 16 A/cm is required in one direction (for resetting the Wiegand wire) and a field strength of about 80 to 120 A/cm is required in the opposite direction. It is typical of the Wiegand effect that the amplitude and width of the pulses which are generated are substantially independent of the rate of change of the external magnetic field and that they have a high signal-to-noise ratio.
The invention is also applicable to other bistable magnetic elements, which have two magnetically coupled regions which differ in hardness (coercive force), and like Wiegand wires can be used to generate pulses by an induced quick reversal of the direction of flux in the soft magnetic region. For instance, German Patent Specification No. 25 14 131 discloses a bistable magnetic element in the form of a wire having a hard magnetic core, e.g., of a nickle-cobalt alloy, an electrically conductive intermediate layer, e.g., of copper deposited on the core, and a surface layer, which has been deposited on said intermediate layer and consists of a soft-magnetic material, e.g., of a nickel-iron alloy. In another embodiment, the core consists of a magnetically non-conducting but electrically conducting metallic inner conductor, which has a high reluctance and consists, e.g., of a beryllium-copper alloy and on which first the hard magnetic layer, then the intermediate layer and finally the soft magnetic layer have been deposited. The pulses generated by such a bistable magnetic element will be smaller than those generated by a Wiegand wire.
Another bistable magnetic element consisting of two layers has been disclosed in the periodical ELEKTRONIK. 9, May 6, 1983, on pages 105 and 106.
Patterns of Wiegand wires can be used in binary encoding of information if two rows of parallel Wiegand wires are provided on a carrier in such a manner that the two rows are offset for each other in the longitudinal direction of the Wiegand wires and that each space between adjacent Wiegand wires of one row is in register with a Wiegand wire of the other row. If the carrier provided with the two rows of Wiegand wires is moved past a reading head or if a reading head is moved past the carrier in the longitudinal direction of the two rows, each Wiegand wire moving past the reading head will generate an electric pulse in a detector winding of the reading head. The reading head will distinguish between pulses generated by the Wiegand wires of the first and second rows and will associate the values "0" and "1", respectively, with said pulses.
The reading heads which are known for the purpose stated are intended foruse with Wiegand wires which are asymmetrically excited. In the reading head disclosed in Laid-open German Application No. 32 23 924 the electric detector winding surrounds an intermediate leg of an E-shaped core, which is divided by a nonmagnetic intermediate layer into two soft magnetic E-shaped core elements, which are parallel to each other.
In the prior art and within the scope of the invention the spacing of the three legs of the E-shaped cores, the length of the Wiegand wires provided in a carrier and the offset between the two rows of Wiegand wires provided in that carrier are so selected in view of each other that the two outer legs of the E-shaped core will be close to the outer ends of the Wiegand wires of the two rows thereof if the reading head is moved with the backbone of the E-shaped core in parallel to the Wiegand wires and the free ends of the legs of the E-shaped core face the Wiegand wires. In that case the intermediate leg will be close to the inner ends of the Wiegand wires of the two rows (see FIG. 1) and the short-time change of the magnetic flux which accompanies the reversl of the direction of flux of a Wiegand wire of one row and of a Wiegand wire of the other row will result in an induction of pulses having different polarities in the detector winding.
In the use of the known reading head the asymmetrical excitation of the Wiegand wires required for the generation of pulses is effected by means of two short permanent magnets, which are respectively disposed "above" the upper leg and "below" the lower leg of the E-shaped core and the normals on the pole faces of said permanent magnets include an angle between 60 and 80 degrees with the plane containing the three legs and the back of the E-shaped core and are parallel to the end faces of the three legs of the E-shaped core. (The words "above" and "below" relate to a vertical orientation of the Wiegand wires). As a result of the inclination of the permanent magnets, the magnetic fields established on the two sides of the E-shaped core have mutually opposite directions of flux and different strengths. the weaker of said fields is used for the magnetic resetting of the Wiegand wires and the field having the opposite flux direction is used to trigger the Wiegand wires. The two inclined permanent magnets magnetize the two E-shpaed core elements in mutuallly opposite directions so that the core elements constitute low-reluctance paths by which the magnetic flux is concentrated and confined at the free ends of their three leg. For the also required saturation in a magnetic field having the same flux direction as the magnetic field required for triggering, the known reading head comprises an additional permanent magnet, which is described as a saturating magnet and is spaced a few centimeters, typically 3 to 4 centimeters, from the E-shaped core so that the fields established by the saturating magnet and the other two magnets do not excessively weaken each other. The magnets are so arranged that the magnetic field for resetting the Wiegand wires is disposed between the magnetic fields for saturating and triggering the Wiegand wires, which latter two magnetic fields have directions of flux opposite to that of the resetting field and the latter is virtually confined by the two outer fields. As a result, the minimum field strength required for magnetically resetting the Wiegand wires is exceeded only in a region which is short in the reading direction. The length of that region could only be increased by an increase of the spacing of the magnets and of the overall length of the reading head.
A disadvantage of the known reading head resides in that it has anyway a larger overall length of at least 5 centimeters in the reading direction, which is the direction in which the reading head is moved past the Wiegand wires or the Wiegand wires are moved past the reading head, and that the reading had is susceptible to changes of the distance between the Wiegand wires and the reading head. Even changes of said distance by a few tenths of a millimeter may prevent a generation of Wiegand pulses. For this reason the known reading head is also susceptible to a canting of the reading head on an axis which is parallel to the backbone of the core. The permissible canting angle of the known reading head is very small.
Even a small canting of the reading head may prevent a generation of Wiegand pulses. That susceptibility of the known reading head to changes of its distance from the Wiegand wires and to a canting relative to the rows of Wiegand wires precludes the use of said reading head as a hand-held reading head, and the large overall length of that known reading head involves a relatively large reading movement. A large reading movement will be undesirable, e.g., when cards provided with encoded information in the form of patterns of Wiegand wires must be inserted into a slot of apparatus by which the cards are to be read. In that case the length of the card must exceed the length of the rows of Wiegand wires on the card at least by the overall length of the reading head.
Laid-open German Application No. 32 23 924 describes also a reading head for use with only one row of Wiegand wires. That reading head differs from the reading heads for use with two rows of Wiegand wires essentially only in that a U-shpaed core rather than the E-shaped core is used, the electric winding is carried by the backbone of the core, and the free leg ends of said core are disposed in the reading surface and face the ends of the Wiegand wires moved past the reading head. Such known reading heads have a U-shaped core have the same disadvantages as the above-described reading head having an E-shaped core.
The invention relates also to a reading head for the magnetic scanning of one row of juxtaposed parallel Wiegand wires or other elongate bistable magnetic elements, which reading head comprises a U-shaped soft magnetic core having two parallel legs, which have free ends disposed in the reading surface of the reading head and are connected at their opposite ends by a backbone disposed behind the reading surface of the reading head, and an electrically conductive detector winding surrounding said backbone, said reading head also comprising two short permanent magnets, which are spaced behind the reading surface and have relatively large pole faces and have the same flux density at said pole faces, one of said permanent magnets being spaced above the upper leg of said core and the other of said permanent magnets being spaced below the lower leg of said core, said permanent magnets having a direction of magnetization which is approximately parallel to the reading surface of the reading head, said permanent magnets being so arranged that the magnetic field established between them has substantially antiparallel directions of flux which are approximately parallel to said backbone on opposite side of said core.